Yamaha 1999 15hp 4 Stroke Misfires

[Old Fishing Guy]

I recently purchased a 1999 Yamaha 15hp 4 stroke (carburetor). Motor started and ran great on muffs, so I purchased it knowing that on muffs is not the same as running, in the water, under load, WOT, on a boat. I put it on my boat and did a test run. I was not able to run it WOT due to carb leaking very bad and misfiring a WOT. I replaced the carburetor with a new one, replaced spark plug with NKG Yamaha specified plugs and replaced the coil pack w/ attached spark plug wires. That fixed the carb leak but not the misfire at approximately 3/4 throttle. I put an inductive tach on one of the plug wires and ran the motor on the lake at the highest speed the motor would run smooth, approximately 2800 rpms. Yamaha specifies WOT of 5500 - 6000 rpm. So my thought is the rev limiter is not the problem.

Any thoughts or direction I should go with the troubleshooting?

Thanks in advance,
Old Fishing Guy

 

[racerone]

Check operation of the valves and valve clearance.-----Costs no coins to do so.

 

[Old Fishing Guy]

Thanks racerone. I will look into the valves. I do have an electronic copy of the service manual. Hopefully it will guide me thru the process.

 

[racerone]

Running on 1 cylinder?

 

[Old Fishing Guy]

Not sure. I will need to go to the lake and run it on one cylinder, dis connect one plug wire at a time